Exothermic hot top



United States Patent 3,326,273 EXOTHERMIC HOT TOP Edward John Jago,Ber-ea, and Ronald W. Ruddle, Rocky River, Ghio, assignors to FosecoInternational Limited, Birmingham, England, a British company NoDrawing. Filed Dec. 28, 1965, Ser. No. 517,132 Claims. (Cl. 164349) Thisinvention relates to the provision of linings for moulds used to makeingots or castings from molten metal, and for hot tops, risers and thelike used with such moulds. It further relates to the new compositionswhich are employed in the production of such linings.

In the production of ingots and castings from molten metal it isnecessary to provide that molten metal may feed to the body of the ingotor casting to compensate from the shrinkage which occurs on coolingsince otherwise the ingot or casting may be formed with internalcavities or fissures. The usual method is to provide that thesolidification of the head metal in an ingot mould, or in a hot topprovided thereon, or in the risers and feeder heads of a casting mouldis delayed, so providing a reservoir of molten metal which may feed tothe ingot or casting proper. This delay may be achieved by setting up abarrier to the loss of heat from the head metal by lining the head ofthe ingot mould or the hot top, risers, feeder heads and the like with arefractory heat-insulating composition, or by using a composition ofwhich the ingredients are ignited by the heat of the molten metal toreact exothermically.

In recent years there have come into use, for the production of liningsfor the inner molten-metal-contacting surfaces of metal casting moulds,or of a hot top for such a mould, shaped bodies or linings made ofcastable compositions which contain predominantly a refractory fillermaterial, usually with minor amounts of an organic fibrous material andof a binding medium. The organic fibrous material is usually a paperpulp, e.g. rep-ulped old newsprint. The binding medium may be based onany of a wide variety of materials, e.g. a natural or synthetic resin orglue, elg. a silicone resin, ureaor phenol-formaldehyde resin, acellulose glue, sulphite lye, or sodium-silicate.

The refractory material used in the said compositions is generally asiliceous material such as sand, quartz, quartzite, inorganic silicate,or may be a material such as dolomite. The refractory material may alsoinclude a fibrous refractory, e.g. asbestos, glass fibre, or rock wool.

It has now been discovered that valuable advantages can be obtained byincluding in compositions as aforesaid, in replacement of the whole orpart of the refractory material, ballmill dust as hereinafter defined.

According to the present invention, therefore, there is provided acomposition suitable for lining the inner surface of a casting mould forcasting metal or of a hot top for such a mould which comprises apredominant amount (i.e. at least about 50%) of ballmill dust,advantageously together with about 2 to 30% by weight of organic fibrousmaterial, about 1 to by weight of a binding medium and optionally fromabout 1% up to 10% fibrous refractory material. Preferred compositionsare those containing 78 to 94% ballmill dust, 3 to 9% of organic fibrousmaterial and 1 to 8% of binding agent. The organic material and thebinding agent may be any of those referred to above or mixtures thereof.

It is found that important advantages flow from the use of ballmilldust. It is very cheap material and since it contains a relatively largepercentage of alumina it is substantially as effective a refractory assilica or silicate, but presents no silicosis hazard.

Moreover, since it contains finely divided aluminum metal this tends toburn exothermically when linings of the composition are contacted by themolten mould metal. The compositions being thus self-heating, they tendto improve the feed characteristics of the molten metal with which theyare used. It is to be noted in this connection that various ingredientspresent in ballmill dust, e.g. halide salts, may tend to facilitate thecombustion of the aluminum.

A particular advantage of the hot tops or risers of the invention isthat they are highly exothermic without requiring the presence ofchemical oxidizing agents. Heretofore it has been the practice whenformulating aluminumcontaining exothermic compositions to includesubstantial amounts (e.g. 5 to 30%) of an oxidizing agent such as ironoxide, manganese dioxide, or an inorganic nitrate or chlorate. Thepresent invention, by eliminating the need for such agents, avoids thepossibility of iron or manganese altering the molten metal composition,and reduces moisture absorption caused by deliquescent nitrates orchlorates.

In the method, products and compositions of this invention, use is madeof the material known commercially and industrially as ballmill dust.

Ballmill dust is obtained from the skimming and drosses formed duringthe metling of aluminum and aluminum alloys in an oxygen-containingatmosphere. Usually the skimmings and drosses pass to the secondarymelters for pulverizing by ballmilling or grinding. In some cases thedross may need to be reduced in size in a jawcrusher but generally it issufliciently fine for ballmilling Without any pretreatment. Afterballmilling it is usual to screen the residue. The coarse material(normally +10 or +16 mesh) contains most of the metallic aluminum and isremoved for remelting. The fine material, which is called ballmill dust,may be washed by the producer in order to remove water-soluble salts.

The dross usually is composed mainly of aluminum oxide (resulting fromthe oxidation of the molten metal) and particles of aluminum or aluminumalloy, together with a few percent each of metallic contaminants such ascopper, silicon, iron, zinc, magnesium, and/ or their compounds. Somesilica is generally present, as are fluorides and chlorides of sodium,potassium, and/or other metals (from fiuxing ingredients and theirvarious reaction products). Aluminum nitride is also usually present,resulting from the reaction between aluminum and atmospheric nitrogen.

Generally the fluxes used with aluminum or mixtures containing one ormore of the following components: sodium fluoride, sodium chloride,sodium sulphate, potassium chloride and cryolite.

The ballmill dust may contain up to 50% sodium chloride and values of 10to 15% total fluorides (Water-soluble and water-insoluble) have beennoted.

Sodium aluminate, sodium carbonate and the oxides of the alloyingelements are also often found.

Generally speaking the less developed the aluminum industry in aparticular country, the higher quality the ballmill dust available init, e.g. there is a considerable quantity of ballmill dust containing upto 40% aluminum available in Spain. This is due to both the limited useof fluxes, leading to higher aluminum contents and low chloride andfluoride con-tent of the dusts, and to restricted refining capacity. "Inthe United Kingdom any ballmill dust is refined to extract aluminummetal if its metal content exceeds 30%. By comparison, ballmill dustscontaining over 60% metallic aluminum are by no means uncommon in otherEuropean countries.

The residual aluminum content of ballmill dust depends therefore on thesource and on the type of processing it receives but normally is between10 and 30%. It may however contain as little as 5 or as much as 60 or70% metallic aluminum. For optimal exothermic performance when pouringferrous metals, it is preferred that the ballmill dust contain fromabout 5 to about 45 weight percent aluminum metal (e.g. about to andaccordingly it may in some instances be desirable to fortifyaluminumlean dust with blown or ground aluminum metal. With non-ferrousmetal casting, a higher aluminum content may be desirable.

It is to be understood that the term ballmill dust used herein means aproduct as thus defined.

The composition of the ballmill dust preferably used in the practice ofthe invention may vary widely, e.g. within the ranges shown below:

Percent Aluminum 5 to 70 Aluminum oxide 15 to 60 Zinc 0 to 5 Chloride 0to lead 0 to 1 Most preferably, however, the composition containsapproximately 15-45% aluminum, about 1% chloride and not much more than0.1% each of zinc and lead. A minimum aluminum content of 15% seems toprovide a suificiently intensive reaction to make feeding of steelcastings efficient in riser sizes down to about a 3 inch diameter.However, improved efficiency can beobtained by using higher aluminumcontents in the feeding of both ferrous and non-ferrous alloy castings.Generally speaking, the advantage of higher aluminum contents is mostmarked with the smaller diameter risers, but advantage is to be expectedwith all sizes.

Impurities such as zinc, chloride, and lead are detrimental in that theycause the evolution of excessive or toxic fumes during combustion of theproduct. They do not eifect the resultant castings however. The presenceof some fluoride is beneficial since it increases the sensitivity of themixt-ure; for example, the addition of 2% sodium fluoride /2% in thefinished product) results in improved burning especially on smalldiameter risers.

The 'ballmill dust should preferably not contain large particles ofdross since, if included, these tend to increase the density of thefinal product, and reduce the efficiency of the aluminum combustion.

Typical component and sieve analyses of ballmill dust are given in thetable below. The samples are identified as follows:

Available in United Kingdom.

TAB LE I.TYPICAL ANALYSIS OF BALLMILL DUSTS Sample A B O D EComposition, wt.

percent:

A 22. 34 10. 00 31. 00 9. 40 0. 75 0.50 1. 00 0.50 0. 10 0. 10 0. 10 0.10 1.30 0. 10 1. 70 0. 45. 25 24. 35. 28 41. 43 5. 50 2. 0O 2. 62 8. 960. 0. 21 0.98 0. 35 1. 15 1. 89 4. 28 0. 89 0. 13 Nil 2 86 2. 83 15. 403. 01 0.70 0.80 1. 4. 00 0.75 0.75 0.77 0. 98 0. 55 O. 50 0. 39 0. 64 1.73 48. 9 2. 34 14. 76 0. 10 2. 5O 0. 30 1. G6 0. 02 0. 10 0. 63 3. 54 1.16 7.25 Trace Truce TABLE II.-TYPICAL SIEVE ANALYSIS OF BALLMILL DUSTSSample A l B l o Sieve Analysis:

Ballmill dust particle size distribution depends both on the extent ofgrinding and on the screens used to recover the aluminum. It ispreferred however that more than half, optimally more than 95%, of thedust pass a 10 mesh screen, and optimally more than pass a 20 meshscreen.

The compositions of this invention are preferably preformed as slabs orsleeves for use in lining the head of an ingot or casting mould or inlining a hot top for a metal casting mould.

An especially useful shape in the practice of this invention is that ofa slightly coned sleeve having at its narrower end a wall across itprovided with a vent, substantially the same shape as, for example, aninverted flowerpot. In normal foundry practice it is customary, whenusing exothermic riser sleeves or sleeves of refractory insulatingmaterial, to cover the top layer of molten metal, after pouring, withexothermic or refractory heat-insulating powder. This method has severaldisadvantages, such as inhomogeneity and inconvenience to the user, andheat loss due to the splitting of the powder layer during cooling.However, fabrication of sleeves in the shape indicated above eliminatesthe need for powder, increases the insulation of the top metal and makesthe casting operation easier. In addition, sleeves thus formed arestronger and easier to store.

The slabs or sleeves may conveniently be formed by a slurry technique asfollows: The ingredients of the composition are made up to an aqueousslurry. Advantageously, a small quantity of a surfactant known per se isincluded in the composition to facilitate this. The slurry is chargedinto a vessel having a perforate wall or walls and pressure or vacuum isapplied to cause the slurry to be urged against the perforate walls. Theliquid me dium of the slurry passes through the perforate walls aseflluent and the solid constituents are compacted against the perforatewalls as layers of desired thickness.

One of the most convenient methods of effecting this process when makingsleeves is to spin a porous mould containing the slurry at high speed,thereby to drive the Water from the slurry; the mode of action is thatof a normal domestic spin-dryer. By this method, the internal diameterof the sleeve formed may only be controlled by the amount of slurryadded to the mould, and the solids content thereof. In this connectionthe solids content of the slurry is preferably to 50%, most preferably30 to 35%. Typical spinning times and speeds are 1200 to 1400 r.p.m. for1.5 to 3 minutes. The compacted sleeves may be withdrawn from the mould,and are fairly easily handleable.

As indicated above, vacuum forming processes for slabs and sleeves arealso practical. However, economic considerations usually limit the useof this method to small slabs or sleeves. Water extraction times tend tobe longer than with spin-forming. It is preferred to use a slurry of tosolids content, lower concentrations of solids requiring longer waterextraction times. This method is of great value where, for example, theinternal diameter of a sleeve must be formed to close dimensionaltolerances.

Before use the slabs or sleeves must be dried. This is generallyeffected by drying on vented core plates in normal ovens through whichair is passed.

It must be appreciated that where the foregoing technique is employedany binder which is soluble in the liquid medium will be largely lost inthe effluent (from which if desired it may be recovered) and it istherefore necessary to employ sufiicient binder to ensure that enough ispresent in the liquid which is retained by the compacted solidcontsit-uents to provide the necessary composition as earlier described.However, a water-insoluble, thermo-softening binder may be employed insolid powder form (e.g. phenolic resins in a suitable state ofpolymerization). In such case losses with the efiluent will benegligible. A mixture of insoluble and soluble binders may be employed.

Further, it is desirable that the ballmill dust used contain not morethan 50 to 60% of material of particle size 200 mesh (Tyler) sinceslurries made using dust with these or even higher fine dust contentstend to be nonporous and tend to require longer spinning or compactingto extract the water. Where long water extraction times are acceptable,however, materials containing up to 85%, or over, of 200 mesh particlesmay be used.

Permeability in formulations containing unwashed ballmill dust may beincreased by incorporating a small amount, e.g. 1 to 5% of Wood flour inthe mix. This amount brings the permeability to at least about 4 or 5A.F.S. units in order to avoid blowing during metal pouring. Thepreferred range of permeabilities is about 812 A.F.S. units.

The following specific example will serve to illustrate the invention:

Example The following Were mixed:

Parts by wt.

Ballmill dust 87.70 Urea formaldehyde resin solids 1.25 Phenolformaldehyde resin solids 2.50 Asbestos fibre 1.75 Cellulose fibre(scrap newsprint) 6.50 Surfactant (Arquad 2C-75, a dialkyl quaternaryammonium salt) 0.30

and water added to give a slurry of solids content approximately 33%.This slurry was formed into slabs and sleeves of varying sizes, andthese were then dried in ovens. The sleeves were used to line risercavities in the manufacture of said castings and gave excellent feeddown to 2 inch internal diameter risers, with the formation of muchreduced pipe, as compared with the use of prior refractoryheat-insulating materials.

A similar formulation which included 0.5 by weight of sodium fluoride(replacing 0.5% of ballmill dust) gave adequate feed in risers down to 3inch internal diameter. In like manner, calcium fluoride or cryolite maybe used, each in amounts of 0.1 to 2%.

A major advantage of the invention is that the exothermic refractory ofthe invention becomes a relatively soft, frangible material aftercompletion of the exothermic reaction. It thus may easily be removedfrom the solidified metal in the mould cavity, allowing facile recoveryof the riser metal and clean rolling of hot topped ingots.

Additionally, the compositions are essentially nonsmoking, and may beused without discomfort to near-by workers.

We claim as our invention:

1. As a new article of manufacture, an exothermic hot top, riser, or thelike having a molten-metal-contacting surface comprising a shaped bodycomposed dross obtained by the atmospheric oxidation of molten aluminummetal, said dross having a composition comprising from about 5 to aboutweight percent aluminum metal and from about 15 to about 60 weightpercent aluminum oxide and being of a particle size which will passthrough a 10 mesh screen, said shaped body being substantially free ofoxidizing agents and from about 2 to about 30 weight percent of anorganic fibrous material.

2. The article of claim 1 wherein said shaped body contains from about 1to about 10 Weight percent of an inorganic fibrous material.

3. The article of claim 1 wherein said shaped body contains from about 1to about 10 Weight percent of a binding agent.

4. The article of claim 1 wherein said shaped body contains from about84 to about 92 weight percent of said finely pulverized dross.

5. The article of claim 1 wherein said shaped body comprises about 78 to94 weight percent of said dross, about 3 to 9% of organic fibrousmaterial, and about 1 to 8% of binding agent, wherein all of said drosspasses through a 20 mesh screen and not more than about 60% passesthrough a 200 mesh screen, and wherein said shaped body has an A.F.S.porosity of at least about 4.

References Cited UNITED STATES PATENTS 2,390,500 12/ 1945 Charman et a1.22-1 2,500,097 3/1950 Soifel 221 2,891,293 6/1959 Forsythe 221 3,039,1586/1962 Mueller 221 3,171,173 3/1965 Ingala 24962 3,212,749 10/1965Labate 249- 200 I. SPENCER OVERHOLSER, Primary Examiner. E. MAR,Assistant Examiner.

1. AS A NEW ARTICLE OF MANUFACTURE, AN EXOTHERMIC HOT TOP, RISER, OR THELIKE HAVING A MOLTEN-METAL-CONTACTING SURFACE COMPRISING A SHAPED BODYCOMPOSED DROSS OBTAINED BY THE ATMOSPHERIC OXIDATION OF MOLTEN ALUMINUMMETAL, SAID DROSS HAVING A COMPOSITION COMPRISING FROM ABOUT 5 TO ABOUT70 WEIGHT PERCENT ALUMINUM OXIDE AND BEING OF A PARTICLE SIZE WHICH WILLPASS THROUGH A 10 MESH SCREEN, SAID SHAPED BODY BEING SUBSTANTIALLY FREEOF OXIDIZING AGENTS AND FROM ABOUT 2 TO ABOUT 30 WEIGHT PERCENT OF ANORGANIC FIBROUS MATERIAL.